Climate change is affecting wind energy's potential to provide power, diminishing a renewable energy source that scientists and policymakers are counting on to replace fossil fuels, according to two scientific studies.

The world is turning more and more to renewable sources of energy - wind, solar power, and in some cases energy from flowing water - to fight climate change. But what if climate change itself alters the distribution of wind, or sunlight falling on the Earth's surface, or river flows, and so changes or even shrinks the potential of these energy sources?

The studies suggest that, at least for wind energy, that is not only happening - at least in some key locations - but that it could grow worse.

"Renewables, including wind, are an important part of many nations' and even states' overall strategies for reducing greenhouse emissions," said Kristopher Karnauskas, a researcher at the University of Colorado, and first author of one of the studies. "So it's important that we fully understand how the potential efficacy of that mitigation strategy may be changing concurrently with the problem itself. We can't assume that the baseline wind energy resource is a constant."

Why would wind energy potential change because of climate change? At the most fundamental level, winds are driven by the unequal distribution of the sun's energy across the surface of the Earth, which in turn creates regions of different atmospheric pressure. Wind then flows from regions of high pressure to regions of low pressure. Those installing wind turbines try to place them in spots where that flow tends to be particularly strong.

But climate change is also distributing energy unevenly across the planet. In particular, land surfaces are warming up more rapidly than are ocean surfaces, even as the Arctic is warming up much more rapidly than the northern hemisphere's middle latitudes.

It's not surprising that the distribution of winds around the Earth would change under global warming. Indeed, in the Southern Hemisphere, some researchers believe that changing winds are responsible for the growing melting of Antarctic glaciers, as they are driving warmer, deeper waters closer to the icy continent.

So how could changing wind patterns affect the deployment and effectiveness of wind energy?

The first of the two studies, recently published in Nature Scientific Reports, gives a first glimpse at an answer. It finds that the nation that has installed more wind energy than any other on Earth - China - is actually seeing a lowering of wind energy potential across vast regions, especially inner Mongolia and Gansu, two of the largest installation areas.

"To my great surprise instead of finding a random signal, we found that it was actually declining," said Michael McElroy, a Harvard Earth sciences professor who is one of the authors of the study. He conducted the research with Peter Sherman, the first author, and Xinyu Chen of Harvard.

The researchers found, based on a large database of meteorological records, that there already has been a wind energy decline in key regions of China from 1979 through 2015. And they found that this had happened in concert with an overall warming trend, although natural climate fluctuations also played a role.

The change occurred, McElroy said, because the Asian continent has been warming up faster than the Pacific Ocean offshore.

"The circulation of the atmosphere over East Asia is what's called a monsoonal circulation, driven by the temperature contrast between the land and the sea," he said. "In winter, that temperature contrast is trending down."

China, the world's largest emitter of carbon dioxide by a considerable margin, has already installed well more than 100 gigawatts of wind energy, considerably more than any other nation. So the new research plainly shows how climate change can render itself harder to fight.

"This is a very important result because it highlights the many negative impacts of climate change," Dan Kammen, an energy policy expert at the University of California at Berkeley, said of the China study, with which he was familiar but not involved.

These changes in China already seem to be happening. But the second study, published Monday in Nature Geoscience and apparently the first comprehensive look at wind energy resources under climate change, projects forward to the future using 10 climate change models. And it finds more of the same across the mid-latitudes of the Northern Hemisphere - a key area for wind energy installation from the United States to Europe to China - even as under some scenarios, the models show a growth of wind resources in the tropics and the Southern Hemisphere.

"What we found is that global warming will reduce the wind energy resource across much of the Northern Hemisphere, especially in the central U.S.," said Karnauskas, who conducted the work with university colleagues Julie Lundquist and Lei Zhang.

Based on high and low scenarios for the volume of greenhouse gas emissions, Karnauskas's study found declines on the order of 10 percent to 40 percent of potential wind energy in the Northern Hemisphere middle latitudes by 2100. For the central United States, it showed declines by 8 percent or 10 percent by 2050, depending on whether the world follows a lower or higher greenhouse gas emissions scenario.

Meanwhile, wind energy potential in Southern Hemisphere regions such as East Brazil, West Africa and East Australia increased significantly in the models, but only under high emissions scenarios. That's a trade-off, but hardly a simple one: The majority of current wind installations are in the Northern Hemisphere, as is the majority of global population and thus, naturally, global energy demand.

The changes in wind energy potential found in Karnauskas's study had a different cause than in McElroy's - at least for the Northern Hemisphere. This was the reduction in the temperature difference between the Arctic and the mid-latitudes, as the Arctic warms up at a considerably faster rate. This changing temperature gradient is already being blamed by some climate scientists for altering mid-latitude weather.

It's important to emphasize that in no case are we talking about a vanishing of the wind resource - rather, it's a fractional decline. Turbines will still draw energy from the winds, but the overall potential could be less.

"The reduction over that 40 year period is roughly 10 percent," said McElroy, referring to what has already occurred in China. "That's a big deal, but it's not a game changer."

Furthermore, these studies looked at wind energy installations on land surfaces - not coastal offshore installations, which are growing in number, or true deepwater installations, which are only beginning at the moment. These could present a different story.

Nonetheless, the new research underscores that, if we are going to rely on the natural processes of the Earth to counter the ways in which we're changing the Earth, we should be ready for the occasional curveball.

"I don't think that it says, let's not employ wind energy as part of the solution to climate change," said Karnauskas. "It's always been cast as one piece of the pie that makes up a strategy to reduce emissions. But it also means that it is precariously sensitive to the climate state itself."